There are 2 vents exiting from 2 adjacent bathrooms. One exit looks ok, as for the other one I noticed building snow (condensation) - see picture. Left is from Dec 15, Right Dec 24.

While the exterior temperature was unusually low for these days I don't understand the phenomenon. Is there any explanation? Can anyone help!

Thank you, kindly.

enter image description here

  • 1
    What problem do you think you need to solve? (By the way, this exact phenomenon occurred on my beard while skiing yesterday.)
    – isherwood
    Jan 24, 2022 at 19:21
  • Are both bathrooms used for showering? Do the flaps move much when either fan is on?
    – rtaft
    Jan 24, 2022 at 19:40

1 Answer 1


The bathroom vent fan is taking warm, moist air and expelling it from your bathroom to prevent mold, condensation and rot.

When that moist air hits the cold exterior air, the moisture condenses rapidly and forms frost. The same thing happens on your windshield (if you park outside) on cold nights.

This bit of frost:

  1. lets you know that your bath exhaust is actually doing its job
  2. is absolutely harmless to your house

Either that, or Casper the Friendly Ghost got drunk and ran into the wall instead of floating through it.

  • Thank you for your expanations. Have a great day! Jan 24, 2022 at 20:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.